It is no longer true. The newest version of Wi-Fi Calling from T-Mobile does not require radio level interaction. As long as you can switch off the cellular radio completely when in Wi-Fi, this new version will work fine.

In fact, the new Wi-Fi Calling solution from T-Mobile uses IMS instead of UMA, which means it is entirely in software (and does not rely on GAN radio calls in the RIL). It is also mostly open source already. Back in December, T-Mobile and Movial (the company that T-Mobile partnered with to develop the new solution) open sourced the entire stack used for Wi-Fi Calling. For someone who understands the Android software, it shouldn't be difficult to integrate and bring in support for the new version of this feature into AOSP and then have it added to the CM9 source code.

Oh yes, it is also going to be used for T-Mobile's VoLTE for its 2013 LTE launch, too. So it really does need to become supported in CM9 and AOSP soon.

Well, here's the thing. Wi-Fi Calling is no longer an "app" in all phones released after October 2011. The new IMS Wi-Fi Calling system is fully integrated into the IMS Service system, and the only part of it exposed to the user is a menu in the Networks settings for turning it on and off.

It doesn't use Kineto's stuff anymore. While Kineto's stuff still works, it isn't compatible with Android 4.0, so you cannot use that very well on ICS ROMs. You have to use the IMS version.